The present invention relates generally to an electrical bus with a means for dissipating heat from semiconductor components, and more particularly is a module comprising an electrically conductive porous metal heat sink that has semiconductor components attached to a surface thereof, the semiconductor components being directly and metallurgically attached via soldering to the heat sink. The invention also includes the method of manufacturing a porous metal heat sink for an electronic device.
Many electronic components generate heat during operation. This characteristic becomes significant in instances in which an electronic device is used to generate, transfer, or convert electric power. An excellent example of this effect, cited in Applicants"" related application (referenced above), is the inverter used in electric traction motors. Heavy electric vehicles, including locomotives, road and off-road vehicles, are driven by electrically powered traction motors which turn the wheels or tracks of the vehicle. These traction motors operate on AC power, but the electrical power supplied by the energy source of the vehicle is typically DC. This DC power must therefore be converted to AC power in an inverter. Further, the rotational speed of such traction motors is usually controlled by means of the frequency of the AC power. The electric power generation rectification/inversion/voltage control/frequency control system (hereinafter summarized as power converter) requires the use of multiple semiconductor devices, and integrated circuits to control the semiconductor devices, all of which generate a great deal of heat. Many other electrical applications require the use of semiconductor devices and integrated circuits to control them. They therefore also require some means of dissipating the heat generated.
To dissipate the heat produced in a power converter used in an electric vehicle, current art vehicles use either air-cooling systems or water cooled heat sinks, or both in combination. Similarly, other current art electronics applications require some method of heat dissipation, most often air-cooling. The current art methods of cooling give rise to several problems.
For any device to be air-cooled there must be adequate space around the device for air to flow in sufficient volume to remove the heat. In the specific instance of the power converter for the electric traction motor, since traction motor applications typically utilize three-phase AC power, six power semiconductor switch assemblies and six diodes must be employed. The electrical requirements of the motors require that a capacitor bank be present in the power converter, along with the power semiconductors and their accompanying diodes, sensors, switch driver circuitry, etc. The number of components required therefore mandates a significant space requirement. This space requirement is then further greatly exaggerated by the need for space to accommodate flow of air around the power converter components. This space requirement problem is common to all air-cooled electronic devices.
In direct contradiction to the need for open space for the flow of cooling air is the fact that all electrical devices function best in enclosed, non-ventilated environments. This kind of environment reduces the potential for contaminant buildup. Contaminant buildup can not only impede the desired heat transfer, but may also cause an electrical failure of the device. Therefore air-cooling can directly create a situation detrimental to the function of the electrical device.
Because of the problems caused by air-cooling, some current art devices utilize water-cooling, particularly in high power applications, to provide a more controlled environment. But a water-cooled heat sink system suitable for a power converter is generally not readily available in a vehicle. Thus, utilization of a power converter that is water-cooled leads to the necessity of including a water cooling system in a vehicle that would not otherwise require it. Still more space is therefore required. Moreover, due to the conductive nature of water, it is necessary to dielectrically isolate the power converter components from such a water-cooling system and this, in itself, results in less than satisfactory cooling of the semiconductor switches and diodes.
Further problems created by the use of current art cooling methods, in particular by the size requirements demanded by the current art cooling systems for power converters, are due to the fact that the power converter comprises a large unit contained in a compartment dedicated only to the power converter. This necessitates that lead wires for control and feedback systems must be fairly long, typically anywhere from 2 to 10 feet. Longer wires are by necessity heavier than shorter wires, both in terms of weight and electrical rating. Longer wires significantly increase the potential for distorted signals.
High power electric traction motors suitable for heavy vehicles are usually oil-cooled and such vehicles therefore require an oil-cooling system. Utilizing a dielectric fluid such as oil to cool the power converter allows the heat sink to be an integral part of the circuitry of the power converter. The oil-cooled heat sink can thus serve as an electrical power bus to which heat generating electronic components are directly mounted. The enhanced cooling capability achieved by oil cooling the electrical bus greatly enhances the electrical performance of these electronic components and therefore allows for a very compact, high performance power converter. The same oil-cooling system used to cool the traction motors can be used to cool the power converter. The space requirements are thus considerably less than current art air-cooled and water-cooled systems.
Accordingly, it is an object of the present invention to provide a module in which electronic components are attached to an electrically conductive heat sink in a way that gives rise to significantly reduced space requirements.
It is a further object of the present invention to provide a module in which electronic components are attached to an electrically conductive heat sink in a generally planar assembly, thereby reducing inductance.
It is another object of the present invention to incorporate the electrically conductive heat sink as an active part of the component circuitry.
It is still another object of the present invention to allow the electronic components to be metallurgically bonded to the electrically conductive heat sink.
The requirements of the electrically conductive heat sink, and the objects of the present invention are as follows:
1. The heat sink must comprise a plurality of pathways uniformly distributed throughout so as to allow uniform distribution and passage of the cooling fluid.
2. At least one surface of the heat sink must be made from a thermally and electrically conductive metal that is suitable for metallurgical bonding to the electronic components. Other elements of the heat sink can be constructed from metals offering optimum properties for fabricating an electrically conductive heat sink.
3. There must be a sufficient surface-area-to-volume relationship for the internal pathways to provide for the required convective transfer of heat from the heat sink to the heat exchange fluid.
4. All metallic components of the heat sink must be metallurgically bonded to each other to minimize resistance to conduction of heat and electricity throughout.
5. The bonding surface of the electrically conductive heat sink must be prepared to allow for metallurgical attachment of the electronic components.
The present invention is a module comprising an electrically conductive heat sink that has semiconductor components attached to the surface thereof, the semiconductor components being directly and metallurgically attached via soldering to one surface of the electrically conductive heat sink, and the method of manufacturing the heat sink with associated electronic components. One example of an electronic device that can be constructed with the module according to the present invention is an oil-cooled inverter for an electric traction motor. Six IGBT switches and twelve diodes are metallurgically attached to each heat sink, which also serves as an electrical bus, and six such modules are used in the assembly of the 3-phase inverter.
In the preferred embodiment of the heat sink of the present invention, a hollow metal housing is fitted with inlet and outlet ports to allow a cooling fluid to flow through the housing. The housing is filled with a plurality of metal balls metallurgically bonded together at the points of contact so as to provide for both conductive heat transfer from one ball to another and a plurality of fluid flow paths through the interstices between the balls. The metal balls serve as conductive paths to transfer heat and to dissipate the heat into the cooling fluid that flows through the open spaces around the balls. Alternative methods of forming the porous metallic conductive interior element of the heat sink include replacing the metal balls with a machined block of a conductive metal such as copper, brass, bronze, silver, or aluminum, or with metal wool, or with metal felt, or with open cell metal foam, or the like.
Because it is intended to mount electronic components directly onto the heat sink, at least one surface of the heat sink, the mounting surface, must be made from a thermally and electrically conductive metal that has a similar coefficient of thermal expansion to that of the electronic components. In the preferred embodiment molybdenum is used for the mounting surface. Other metals and metal alloys can be used provided their coefficient of thermal expansion is within the range of 4.5 to 10 ppm/xc2x0 K at 23xc2x0 C. Therefore, forming one surface of the heat sink from molybdenum allows the electronic components to be mounted directly to that surface. Since molybdenum, silicon, silicon carbide, and gallium arsenide have similar coefficients of thermal expansion, the expansion and contraction of the heat sink mounting surface will be similar to that of the substrate of the semiconductor chips. Since both elements, the heat sink mounting surface and the chip substrate, expand and contract a similar amount during heating and cooling, the problems of delamination of the chip from the mounting surface or of the chip substrate cracking and breaking due to thermal flexing when the component heats and cools is eliminated.
In order to be susceptible to receiving soldered components, as is a key element of the present invention, the molybdenum mounting surface of the heat sink must be nickel-plated. Nickel plating of both sides of the mounting surface not only allows silicon chips to be soldered with lead containing or lead-free solders to the external side of the mounting surface, but also provides an improved bonding surface for the brazing process used to attach the porous metallic heat transfer media to the internal side of the mounting surface and to complete the assembly of the heat sink. Electrodeposited nickel plating is sufficient for the external surface on to which the electronic components are to be soldered. However, it has been found by the inventors that an electroless process is optimal to properly nickel plate the interior surfaces of the heat sink housing prior to assembly of the heat sink and concurrent attachment of the porous metallic heat transfer media to the internal surfaces of the heat sink housing by brazing. When an electroless process is used for the interior of the housing, it is a simple matter to extend the process to the exterior of the mounting surface as well.
The electrically conductive heat sink can be assembled from separate components selected for design, manufacturing and performance preferences. Such components can include the controlled thermal expansion mounting surface, a heat sink housing body made from a different material to the mounting surface, end caps or manifolds, and a range of alternate porous metallic internal elements. The use of such separate components readily enables the preferred porous metallic internal element to be assembled inside the electrically conductive heat sink housing and manifold connections for heat transfer fluid ingress and egress to be made. After the porous metallic internal element has been placed in the interior of the heat sink housing and the remaining components of the heat sink assembled, all components of the heat sink are joined by brazing. The joints between the components of the housing have minimal mechanical performance requirements, but do have to seal the unit with a sufficiently strong bond so that pressurized fluid can flow through the heat sink without leaks.
For the preferred embodiment of the present invention, the porous metal heat transfer element is constructed by packing relatively small, uniformly sized balls together in the heat sink housing. It has been found by the inventors that balls having a diameter between 0.05 and 0.15 inch are optimal. The balls must be bonded together and the bond contact area must be sufficiently large to allow for conduction of heat and electricity from ball to ball. The metallurgical bonding between the balls is achieved by plating the balls with sufficient volume of a brazing compound so that during a brazing cycle, a sufficient volume of liquid is produced to allow wetting at contact points between the balls. This process increases the size of the conductive heat transfer paths and minimizes thermal and electrical resistance.
In the preferred embodiment, copper balls are used as the porous metal heat transfer element in order to optimize conductive heat transfer, and because copper has the necessary ductility to allow the manufacture, with currently available technology, of balls of the small size required. However, copper alloys are susceptible to hydrogen embrittlement, so the composition of the balls must be controlled to allow for the necessary thermal processing required during brazing. An OFHC (Oxygen Free High Conductivity) copper alloy is selected. The copper balls are plated with silver and copper-silver eutectic brazing compound is formed on the surface of the balls during thermal processing.
The module of the present invention is constructed so as to form a planar bus device. The generally planar structure of the heat sink assembly and its companion electrical components significantly reduces the inductance of the device. This allows the semiconductor components to function more efficiently.
The module of the present invention can be used with many electronic components without affecting the electrical operation of the component. In any semiconductor a portion of the power passing through the chip is dissipated as heat. This becomes significant at currents greater than 100 amps and results in rapid heating of the semiconductor. This heat must be removed to maintain the junction temperature of the semiconductor at or below an acceptable level. Above this temperature there is rapid degradation and failure of the semiconductor chip. One key difference between the module of the present invention and prior art components is that the semiconductor chip is metallurgically attached to the heat sink, that the heat sink is electrically hot, and that the heat sink assembly is used as part of the circuitry of the electronic device. In current art practice semiconductor chips can be mounted to an electrically insulating but thermally conductive plate or can be mounted directly to an air cooled heat sink. In all cases the means of mounting has greater electrical resistivity and lower thermal conductivity than with the present invention. Metal filled adhesives, greases, and gels are used as interface materials when both electrical and thermal conductivity is sought. Silver filled epoxies used for this purpose typically have an electrical resistivity greater than 150 micro-ohm.cm at 23xc2x0 C. and a thermal conductivity of less than 10 W/mxc2x7K at 23xc2x0 C. The low thermal conductivity of current art interface materials implies that the rate of heat transfer from the chip is less than with the present invention and hence that the current flowing through the semiconductor must be maintained less in order to maintain a lower level of internal heat dissipation and, hence, an acceptable junction temperature. In those instances where the chips are bonded directly to a heat sink, and particularly at currents greater than 100 amps, I2R heating will heat the interface. The more resistive current art interface materials will result in greater interface heating than with the present invention and, thus again, result in limitations in allowable current.
One of the chief advantages of the present invention is that it greatly reduces the space required for the electronic components to function efficiently. The heat sink of the present invention accomplishes the same amount of heat transfer as prior art devices having vastly larger size and weight requirements. A traction engine inverter module constructed according to the present invention requires only 10% of the amount of space required by the inverter module of current art devices. The weight of the inverter module of the present invention is also greatly reduced, being only 20% of that of the current art. This tremendous reduction in size and weight leads to the added benefit of making the component far less expensive and more practical to xe2x80x9csparexe2x80x9d, that is, to stock an entire component module for repair use.
Another advantage of the present invention is that the reduced size and compact assembly of the semiconductor components allows them to be mounted closer to the devices they control or support. This significantly reduces the length of the lead wires required for feedback and control systems.
A still further advantage of the present invention when embodied as an inverter is that it utilizes the same coolant supply system as does the electric motor of the vehicle, thereby eliminating the necessity of a water cooling system.
A still further advantage of the present invention is that it provides a molybdenum surface to which electronic components can be soldered. The chips of the component circuitry are mounted directly on the surface of the heat sink, thereby eliminating components and space requirements. One surface of the heat sink is electrically hot, and serves as part of the circuitry. The direct attachment significantly reduces thermal and electrical resistance between the semiconductor components and the heat sink.
Still another advantage of the present invention is that the planar structure achieved for the module reduces inductance.
These and other objects and advantages of the present invention will become apparent to those skilled in the art in view of the description of the best presently known mode of carrying out the invention as described herein and as illustrated in the drawings.